In rotary drilling operations, a drill bit is attached to the end of a drill string which is rotated at the surface by a rotary table. The weight of the drill string causes the rotating bit to bore a hole in the earth. As the operation progresses, new sections of drill pipe are added to the drill string and increase its overall length. Periodically during the drilling operation, the open borehole is cased to stabilize the walls, and the drilling operation is resumed. As a result, the drill string usually operates both in the open borehole and within the casing which has been installed in the borehole.
The power to drill is transmitted through the drill string to the drill bit. The amount of power which can be transmitted is limited to the maximum torque a drill string can sustain.
During the drilling of a borehole through underground formations, the drill string undergoes considerable sliding contact with both the steel casing and rock formations. This sliding contact results primarily from the rotational and straight movements of the drill string in the borehole. Friction between the moving surface of the drill string and the stationary surfaces of the casing and formation creates considerable drag on the string and results in excessive torque during drilling operations. The problem caused by friction is inherent in any drilling operation, but it is especially troublesome in directionally drilled wells. Directional drilling is simply the intentional deviation of a wellbore from the vertical. In some cases the angle from the vertical may be as great as ninety degrees from the vertical. Such wells are commonly referred to as horizontal wells and may be drilled to a considerable depth and considerable distance from the drilling platform.
In all drilling operations, the drill string has a tendency to rest against the side of the borehole or the well casing, but this tendency is much greater in directionally drilled wells because of the effect of gravity. As the drill string increases in length or degree of vertical deflexion, the amount of friction created by the rotating drill string also increases. To overcome this increase in friction, additional power is required to rotate the drill string. In some cases, the friction between the drill string and the casing wall or borehole exceeds the maximum torque that can be tolerated by the drill string and drilling operations must cease. Consequently, the depth to which wells can be drilled using available directional drilling equipment and techniques is limited.
The most common methods for reducing the friction caused by the contact between the drill string and the well casing or borehole rely primarily on improving the lubricity of the drilling muds. It is generally agreed that bentonite helps reduce friction between the drill string and an open borehole. Diesel and other mineral oils are also often used as lubricants, but there is a problem with the disposal of the mud. Other additives include vegetable oils, asphalt, graphite, detergents and walnut hulls, but each has its own drawbacks.
One other common method for reducing the friction between the drill string and the well casing or borehole is to use aluminum drill string because aluminum is lighter than steel. However, the aluminum drill string is expensive, and it is not compatible with many types of drilling fluids (e.g. drilling fluids with high pH).
Still another problem encountered during drilling operations, especially directional drilling, is the wear on the casing and drill string that occurs when the metal surfaces contact each other. This abrasion between metal surfaces during the drilling of oil and gas wells results in excessive wear on both the drill string and the well casing. Presently, the preferred solution to reduce wear of drill strings is to hardface portions of the drill string. A tungsten carbide containing alloy, such as Stellite 6 and Stellite 12 (trademark of Cabot Corporation), has excellent wear resistance. Hardfacing protects the drill string, but it tends to cause excessive abrading of the well casing. This problem is especially severe during directional drilling because the drill string, which has a tendency to rest on the well casing, continually abrades the well casing as the drill string rotates. In addition, some of these hardfacing alloys, such as tungsten carbide, actually make the friction problem worse.